In most fuel supply systems applicable to internal combustion engines, fuel injectors are used to direct fuel pulses into the engine combustion chamber. A commonly used injector is a closed-nozzle injector which includes a nozzle assembly having a spring-biased nozzle valve element positioned adjacent the nozzle orifice for resisting blow back of exhaust gas into the pumping or metering chamber of the injector while allowing fuel to be injected into the cylinder. The nozzle valve element also functions to provide a deliberate, abrupt end to fuel injection thereby preventing a secondary injection which causes unburned hydrocarbons in the exhaust. The nozzle valve is positioned in a nozzle cavity and biased by a nozzle spring to block fuel flow through the nozzle orifices. In many fuel systems, when the pressure of the fuel within the nozzle cavity exceeds the biasing force of the nozzle spring, the nozzle valve element moves outwardly to allow fuel to pass through the nozzle orifices, thus marking the beginning of injection. However, these conventional injectors rely on injector or system components upstream of the nozzle assembly to determine the injection timing, metering and rate shape, and, therefore, may not provide the optimum control over the fuel injection event necessary for certain applications and to achieve certain objectives.
Internal combustion engine designers have increasingly come to realize that substantially improved fuel supply systems are required in order to meet the ever increasing governmental and regulatory requirements of emissions abatement and increased fuel economy. It is well known that the level of emissions generated by the diesel fuel combustion process can be reduced by optimizing the fuel injection timing, metering and injection flow rate for a particular application or set of operating conditions. For example, emissions may be minimized by decreasing the volume of fuel injected during the initial stage of an injection event while permitting a subsequent unrestricted injection flow rate. In other applications, pilot and multiple injections produce the optimal combustion event. As a result, many closed nozzle assemblies have been proposed for enabling more precise control of injection timing, quantity and flow rate throughout engine operation.
One way of more precisely controlling the movement of the needle valve element of a closed nozzle assembly and, therefore, more precisely controlling the fuel injection event, is to utilize a piezoelectric actuator. U.S. Pat. No. 4,649,886 to Igashira et al. discloses a piezoelectric actuator controlled fuel injector where the amount of fuel delivered by the operation of the injector is determined by the driving voltage applied to the piezoelectric actuator. The actuated piezoelectric actuator acts upon a piston which compresses the fuel inside a pump chamber, wherein the compressed fuel is supplied to an injection valve. The reference further discloses that the injection valve includes a needle valve having a step-shaped portion that includes a small diameter portion under a larger diameter portion. The pressure of the compressed fuel acts upon the stepped portion of the injection valve to overcome forces biasing the valve shut thereby raising the needle valve to open the nozzle of the injector. However, the injection fuel, metered by a check valve, is used lift the needle valve to the open position and this metered fuel is then injected from the nozzle. Therefore, the opening of the needle valve element and, therefore, the timing of the injection event is undesirably dependent on the pressure of the fuel to be injected. Moreover, it has been found that the piezoelectric actuators are incapable of effectively and efficiently generating the high fuel pressures desired in many fuel system applications.
U.S. Pat. Nos. 4,728,074, 4,784,102, 4,909,440 and 5,452,858 and PCT Publication No. WO 96/37698 all disclose fuel injectors which utilize a piezoelectric actuator to relieve pressure in a chamber so as to cause a needle valve element to open. For example, U.S. Pat. No. 5,452,858 discloses the use of a piezoelectric actuator to drive a piston which changes the pressure of a working fluid, separate from the injected fuel, in a pressure chamber to control the opening and closing of a needle valve. However, the injectors disclosed in each of these references disclose that the piezoelectric actuator is energized to expand a pressure chamber located adjacent to the injector needle, thus decreasing the pressure within the pressure chamber, in order to relieve the forces biasing the injector needle closed. Also, this injector is not fuel pressure balanced in the closed position and thus the piezoelectric stack be maintained in the expanded state to maintain the hydraulic pressure in the pressure chamber at a high level to hold the element in the closed position.
PCT Patent Publications WO 93/06625 and WO 94/19598 each disclose fuel injection valves using a piezoelectric actuator for moving a piston to controllably vary the pressure of fluid in a hydraulic chamber which is fluidically separate from a fuel supply. The hydraulic chamber is positioned at one end of a needle valve element biased by a spring toward the piezoelectric actuator into a closed position. Pressurization of the fluid in the hydraulic chamber forces the needle valve element into an open position to begin injection of fuel supplied to a nozzle cavity. However, each of these injection systems requires the needle valve to be an outwardly opening valve. Also, the needle valve elements do not appear to be fuel pressure balanced in both positions.
Another way of controlling the movement of a needle valve element is to use a solenoid actuator assembly. U.S. Pat. No. 5,421,521 to Gibson et al. discloses a solenoid actuated fuel injection nozzle assembly including a needle element having an axial passage integrally formed therein for directing fuel, during an injection event, from the orifice end of the assembly to the actuator end. The element is reciprocally mounted in a guide bore section and situated for abutting a valve seat wherein the guide bore section and valve seat have substantially equal diameters for balancing fuel pressure forces. The actuator end of the needle element includes a radial passage for directing fuel from the axial passage to a cavity, surrounding the actuator end, to permit fuel pressure to act on the outer surface of the actuator end of the element so that pressure on the element remains equal at both ends of the element during movement. However, fuel must be drained from the cavity at the actuator end of the element back to the supply which may undesirably result in increased parasitic losses and unacceptable heating of fuel.
U.S. Pat. No. 2,959,360 to Nichols discloses a nozzle valve element having an axial passage formed therein and a cross passage connecting the inner end of the axial passage to the nozzle cavity for diverting fuel from the nozzle cavity into an expansible chamber formed in the nozzle valve element. A plunger is positioned in the chamber to form a differential surface creating a fuel pressure induced seating force on the nozzle valve element to aid in rapidly seating the valve element. This additional differential surface may be equal in area to the additional surface area exposed when the valve has been unseated, thereby providing complete offsetting. However, this valve is not solenoid operated and movement of the valve element is controlled by varying the fuel pressure of the fuel to be injected between a high injection pressure and low pressure. Therefore, this injection valve could not be effectively used with a high pressure common rail system supplying fuel at a substantially constant high pressure level to the valve.
Italian Patent No. 450,866 discloses a closed nozzle injector including a needle valve element having a passage formed therein for directing fuel to a pressure chamber formed by a piston. This arrangement is designed to cause the needle valve element to open during an initial stage, then momentarily close to interrupt injection, and subsequently reopen to continue injection thereby carrying out injection in two separate stages. The fuel pressure in the pressure chamber, formed by a spring loaded piston positioned in the needle valve element, necessarily increases to a high level to cause the closing of the needle valve element and thus the separate stages of injection. However, the surfaces of the valve element in the pressure chamber are sized to create a pressure induced closing force. Moreover, the valve element is actuated by fuel pressure.
Consequently, there is a need for an improved closed nozzle fuel injector assembly operated by an actuator which permits the actuator to effectively, precisely and selectively controlling the rate and degree of opening of a needle valve element independent of fuel pressure.